Discovery of the Early Silurian alkali basalt and its geological implications in northeastern Junggar, NW China

作者：Zhao Hao; Liao QunAn^*; Xiao Dian; Luo Ting; Wang LiangYu; Yin TingWang; Liu HongFei

来源：Acta Petrologica Sinica, 2018, 34(3): 586-600.

• Northeastern Junggar terrane
• Angular unconformity
• Early Silurian
• Alkali Basalt
• Intraplate

A set of the continental volcanic rocks has been recognized in the MoQinulla mountain of northeastern Junggar terrane, which angle unconformably overlies the Middle-Late Ordovician Miaoergou Formation. The volcanic rocks LA-ICP-MS zircon U-Pb dating yield trachytic andesite age of 434. 4 +/- 2. 2Ma, which indicates that it formed in the Early Silurian. The volcanic rocks consist of tephrite, basaltic trachytic andesite, trachytic andesite and pyroclastic rock relatively, belonging to strong alkaline-alkaline series. The volcanic rocks have a SiO2 ranging from 45. 6% to 54.4%, relatively high alkali (Na2O + K2O = 6. 75% similar to 9. 10%), high contents of TiO2 (0. 85% similar to 1. 42%) and Al2O3 (17. 75% similar to 20. 0%), low Me(25. 1 similar to 55. 6), high Rittmann index 8 (3.63 similar to 9. 68) and alkali feldspar and leucite appearing in the matrix. Rocks have a significant differentiation between the light rare earth element (LREE) and heavy rare earth element (HREE), (La/Yb)(N) number ranging from 4. 22 to 6. 29 and unconspicuous Eu negative anomaly. The rocks are all enriched with large ion lithophile elements (LILE), but are relatively depleted in high field strength elements (HFSE), and have a negligible negative Nb-Ta-Ti anomaly. The (Sr-87/Sr-86), values of volcanic rocks are 0. 7033 similar to 0. 7046, with a low positive ENA (t) (+2. 13 similar to +4. 16), showing the depletion mantle source characteristics. But with high alkali, enrichment of LILE and the low ((143) Nd/(144) Nd), values, the rocks reveal the enrichment of source characteristics which may arise from crustal recycling or hybridization of crust materials. Synthesized regional geology and Zr-Zr/Y, Ti/1000-V diagram, we speculate that the Early Silurian alkali basalt formed in the plate extensional setting after the collision between Junggar terrane and Altai-Mongolia terrane, which may represent the stage of Karamaili ocean intracontinental initial extension. It shows that Silurian is a key stage of structural transformation from collisional extrusion to extension in northeastern Junggar terrane.

• 出版日期2018
• 单位中国地质大学（北京）